Societies and Academies

Abstract

LONDON. Roval Society, January 27.— Sir James Dewar: Long-period determination of the rate of production of helium from radium. In a previous communication the rate of the production of helium from 70 milligrams of radium chloride was determined by a succession of observations on the growth of pressure measured by a McLeod gauge. These observations extended over a period of about six weeks. It was thought desirable to make an experiment to determine the amount of helium resulting from this same sample of radium, after standing in a sealed bulb for an extended period. For this purpose the bulb containing the radium chloride was sealed off at the conclusion of the above-mentioned experiment of 1908 and kept for nine months. In order to measure the helium thus produced it was necessary to devise a vacuum-tight joint between the sealed radium bulb and a McLeod gauge so constructed that, after thoroughly exhausting the gauge, the drawn-out end of the radium bulb could be broken off, thus allowing the pressure of the accumulated helium in the radium bulb to be rapidly determined. The total volume of the apparatus was 320 c.c. The pressure in the radium bulb when sealed off at the conclusion of first experiment was 0.00406 mm., the partial pressure due to this amount of helium would be 0.00008 mm., which must be deducted from the observed pressure to get the true pressure due to the helium produced in the radium bulb during the period in which it remained sealed up; also the pressure in the gauge, before breaking (0.00005 mm.), must also be deducted. This gives a corrected pressure of 0.01613 mm., obtained after heating the salt, due to the helium produced from 70 milligrams of pure radium chloride during a period of 275 days, in a space the total volume of which was 320 c.c. The value of the rate in terms of cubic millimetres of helium per gram of radium per day is thus deduced as 0.463.